Flat displays, touch panels, photovoltaics, and next-generation flexible electronics are developed over time, such that a market of transparent conductive films (e.g. ITO) is correspondingly grown. However, the ITO would be partially replaced by other materials due to its high cost, limited sources, and low flexibility. Carbon nanotube (CNT) has properties such as high aspect ratio, high electrical conductivity, excellent mechanical strength, and chemical resistance, thereby having the potential for application in transparent conductive film. Compared to the ITO, transparent conductive film composed of CNT has advantages such as better flexibility, climate resistance, being easier to manufacture, and a natural color.
Carbon nanotube conductive film is generally formed by spray coating, slot-die coating, micro-gravure coating, dip coating, spin coating, vacuum filtration, or layer-by-layer assembly. However, only slot-die coating, micro-gravure coating, and spray coating are capable of mass production. The mainstream of preparing the carbon nanotube dispersion is aqueous solution system. The carbon nanotube of a relatively high concentration is dispersed in water by a dispersant, and dispersants such as sodium dodecylbenzene sulfonate (SDBS) or sodium dodecyl sulfate (SDS) have a higher dispersibility. If an organic solvent such as N-methyl-2-pyrrolidone (NMP), chloroform, dichloromethane, or the likes is selected to directly disperse the carbon nanotube, the concentration of carbon nanotube in the dispersion is too low to form the carbon nanotube conductive film by the slot-die coating or the micro-gravure coating. In addition, the organic solvent may chemically etch some polymer substrate, and the organic solvent will encounter environmental safety problems.
While coating the carbon nanotube as a film, matching the properties of dispersion liquid and the substrate is critical. The surface tension of the carbon nanotube dispersion is limited to the solvent and dispersant systems. Therefore, it is only with difficulty that the carbon nanotube conductive film is formed on a substrate having a relatively low surface energy such as polymethylmethacrylate (PMMA), polypropylene (PP), polyethylene (PE), polydimethylsiloxane (PDMS), polytetrafluoroethene (PTFE), and the like, by coating. In addition, until now it has only been possible to form a conductive film of a carbon nano material on a non-planar substrate by spray coating, and the conductive film still encounters low coating uniformity and a low utility of material.
In comparison of proposed transfer processes of CNT conductive films, transfer technique based on vacuum filtration processes encounters limitations on mass production and preparation of large-area CNT films. Properties of transferred CNT conductive films prepared by dissolving filters and adhesion layer are possibly degraded due to incomplete removal of polymeric materials. In addition, when multiple-step transfer processes are carried out with PDMS stamps, completeness and uniformity of transferred CNT film is hard to control precisely. Accordingly, a novel method of forming a carbon conductive film onto various substrates is called-for.